This invention relates to the selective recovery of copper from various acidic solutions thereof. More specifically, it relates to a continuous process for recovering a pure copper metal product from acidic leaching liquors containing cupric ions admixed with other metal ions and to reagents useful in such a process.
It is known that copper metal can be precipitated from solutions containing complexed cuprous ions by disproportionation of the complex according to the following reactions: EQU 2CuL.sub.2.sup.+ .sub.(aq) .revreaction. 2Cu.sup.+ .sub.(aq) + 4L EQU 2cu.sup.+ .sub.(aq) .revreaction. Cu.degree. + Cu.sup.++ .sub.(aq)
wherein L is a cuprous ion stabilizing ligand, e.g., acetonitrile. U.S. Pat. No. 3,865,744 to A. J. Parker et al. broadly describes various methods of producing and disproportionating cuprous nitrile complexes in acidic solutions. Among the disclosed methods of reducing cupric ions to cuprous nitrile complexes are: reduction with SO.sub.2 gas to Chevreul's salts and dissolution of the latter in the presence of nitrile to give the cuprous nitrile complex; dissolution of crude copper metal with cupric solutions containing nitrile; and reduction of cupric to cuprous nitrile complexes by various metallic reducing agents, e.g., Ag, Fe, Ni, Zn, Cd, Co, Sn, etc.
Another technique utilizing the disproportionation phenomena is disclosed in U.S. Pat. No. 4,038,070 to Rappas et al. entitled Low Temperature and Pressure Continuous Reduction of Copper in Acidic Solutions. In that application, acidic sources of cupric ions are reduced by hydrogen gas in the presence of a hydrogenation catalyst and a nitrile ligand. The presence of the ligand prohibits precipitation of copper metal onto the catalyst by tying up the cuorous ions produced in the stable cuprous nitrile complex. In another embodiment of the invention disclosed in the Rappas et al. patent, acidic solutions of cupric ions are reduced to ligand stabilizing cuprous ions in an electrochemical cell. The cuprous ions produced by both embodiments of the invention of the foregoing application are ideally suited for disproportionation and lead to a copper product of high purity.
The disproportionation technique has many advantages, and the latter method of producing ligand stabilized cupric ion have many features which make the overall process economically attractive. However, there are several areas in the procedure where further improvement is possible. Specifically, under certain conditions, mixing acidic nitrile containing solutions with solutions of cupric ions of rather high concentration can lead to a "salting out" of copper salt, thus producing deposits which can hamper the smooth operation of the process. Furthermore, while catalyst poisoning by precipitation of copper metal on the catalyst is overcome in the Rappas et al. procedure, there is still the possibility that a variety of impurities commonly associated with acidic copper bearing liquors can seriously interfere with the catalytic activity of the finely divided palladium or platinum typically employed. Also, in a commercially successful embodiment, the foregoing procedures would require the use of several catalytic hydrogenation units or several electrochemical cells.
U.S. Pat. No. 3,820,979 to J. Manassen entitled Process for the Production of Metals, discloses a process for obtaining copper, silver, and mercury from aqueous solutions containing these values. In the process of that patent, a quinolic compound dissolved in a water immiscible organic solvent contacts an aqueous solution containing metal values of interest such as copper ions in either the cupric or cuprous state. Such contact results in oxidation of the quinolic compound to a quinonic compound in the simultaneous reduction of copper to copper metal.
In U.S. Pat. No. 4,033,765 to J. Gerlach, entitled Improvements In the Extraction of Copper From Solutions By Reduction With Anthraquinols, certain improvements in this organic copper reduction process are disclosed. One of the discoveries upon which the claimed invention in the Gerlach patent is based is that when a solution of Cu.sup.++ is contacted with a known quinolic reductant, a single electron reduction of Cu.sup.++ to Cu.sup.+ proceeds very rapidly, is essentially quantitative, and unlike the two electron reduction, does not require an excess of extractant. The process disclosed in the Gerlach patent takes advantage of this discovery and provides a considerably improved quinolic organic reduction procedure characterized by reduced reductant losses and increased reaction rates. However, the procedures utilizing the quinolic reductant as disclosed in the Manassen patent and in the Gerlach patent require a series of rather expensive stirred tank reactors as well as at least one relatively difficult three phase separation.